1. Field of the Invention
The invention relates to an enzyme electrode, and to a bio fuel cell that employs an enzyme electrode as at least one of the anode electrode and the cathode electrode.
2. Description of the Related Art
Recently, interest in fuel cells as a countermeasure to environmental issues and natural resource issues is increasing. A fuel cell converts chemical energy directly into electric energy by supplying a fuel and an oxidant to two electrically connected electrodes so that the fuel is electrochemically oxidized. The fuel cell, which electrochemically extracts energy, is not subject to the restrictions of the Carnot cycle, in contrast to thermal electric power generation, and therefore exhibits high energy conversion efficiency. Conventional fuel cells include solid polymer electrolyte fuel cells (PEFC), alkaline electrolyte fuel cells (AFC), phosphoric acid fuel cells (PAFC), etc. Such fuel cells often use platinum (Pt) as a catalyst. However, platinum is very costly, which is an impediment to its widespread use in fuel cells.
Recently, a bio fuel cell is drawing attention as a type of fuel cell that does not use platinum as a catalyst. The bio fuel cell is a fuel cell to which a metabolic mechanism of living organisms is applied. An example of the construction of the bio fuel cell is as follows. First, an oxidoreductase, that is, an oxidation-reduction enzyme, is immobilized on the anode electrode. In order to obtain high output, the bio fuel cell system often contains a coenzyme that is needed for the enzymatic reaction of the oxidoreductase and an electron mediator that receives electrons produced by the enzymatic reaction and transfers the electrons to the electrode. Then, the fuel cell extracts electrons and protons (H+) from a fuel, such as glucose or the like, through the enzymatic reaction. The extracted protons move to the cathode electrode via a proton conductor. In the cathode electrode, protons, electrons and oxygen that is taken in from the atmosphere or the like react to produce water.
With regard to the bio fuel cell as described above, various studies are being conducted in order to increase the power output. For example, Japanese Patent Application Publication No. 2009-140760 (JP-A-2009-140760) describes a fuel cell that includes an anode electrode on which an enzyme, a coenzyme and an electron mediator are immobilized, wherein at least one of the coenzyme and the electron mediator is also added in a fuel solution. Published Japanese Translation of PCT Application No. 2009-515302 (JP-A-2009-515302) describes a fuel cell in which a hydrophobic denatured polysaccharide that is permeable to fuel and an electron mediator is used as a substance for immobilizing an enzyme to the anode electrode.
However, the power output of conventional bio fuel cells is still unsatisfactory. A conceivable reason for this is that in the bio fuel cells, extraction of energy requires a plurality of reaction steps, including an “enzymatic reaction”, an “electron transfer reaction”, etc. The factors involved in these reactions include not only the oxidoreductase but also the coenzyme and the electron mediator, and some of the conditions suitable for the different reaction steps. Thus, optimization for each reaction is difficult.